arXiv:1209.3273v1 [astro-ph.GA] 14 Sep 2012
Astronomy & Astrophysics
manuscript no. tr27˙arxiv
c
ESO 2018
May 25, 2018
Photometric distances to young stars in the inner galactic disk
II. The region towards the open cluster Trumpler 27 at L = 355
o
,
⋆⋆
Gabriel Perren
1
, Ruben A. V´azquez
2
, and Giovanni Carraro
3,4
1
Instituto de F´ısica de Rosario, IFIR (CONICET-UNR), Parque Urquiza, 2000 Rosario, Argentina e-mail:
perren@ifir-conicet.gov.ar
2
Facultad de Ciencias Astron´omicas y Geof´ısicas (UNLP), Instituto de Astrof´ısica de La Plata (CONICET,UNLP), Paseo del Bosque
s/n, La Plata, Argentina e-mail: rvazquez@fcaglp.unlp.edu.ar
3
ESO, Alonso de Cordova 3107, Casilla 19100 Santiago de Chile, Chile e-mail: gcarraro@eso.org
4
Dipartimento di Astronomia, Universita’ di Padova, Vicolo Osservatorio 5, I35122 Padova, Italy
Received May 15, 2012 / accepted May16, 2012
ABSTRACT
Context.
The spiral structure of the Milky Way inside the solar circle is still poorly known because of the high density of the material
that causes strong extinction towards the galactic center.
Aims.
We present results of the first extensive and deep CCD photometric survey carried out in the field of the open cluster Trumpler
27, an object immersed in a region of extremely high visual absorption in the constellation of Sagittarius not far from the Galaxy
center. The survey covers almost a quarter of square degree.
Methods.
We look for young stars clumps that might plausibly be associated with spiral structure. Wide-field UBVI photometry
combined with infrared information allows us to reconstruct the distribution in reddening and distance of young stars in the field
using the Color-Color and Color-Magnitude diagrams
Results.
The analysis of our data, combined too with extensive spectroscopy taken from literature shows that the real entity of
Trumpler 27 as an open cluster is far from being firmly stated. In fact, instead of finding a relatively compact group of stars confined
to a small distance range, we found that stars associated to Trumpler 27 are, indeed, a superposition of early type stars seen along
the line of sight extending over several kiloparsecs beyond even the center of the galaxy. We demonstrate that at each distance range
it becomes possible to generate a color-magnitude diagram resembling that of an open cluster. This way, our analysis indicates that
what was considered an open cluster characterized by a significant age spread is a stellar continuum that reaches its maximum number
of stars at approximately 3.5 kpc from the Sun, the distance of the Scutum-Crux arm approximately. At the same time, and after
analyzing the way early type stars distribute with distance, we found some of these stellar groups may be linked, within the distance
errors, with other inner spiral arms of our galaxy, including the Near 3 kpc arm at approximately 5 kpc from the Sun. However,
very young stars by themselves do not seem to trace strongly the inner spiral arms since they are distributed evenly across several
kiloparsecs toward the center of the Galaxy. This is a remarkable dierence with current HI and CO radio observations maps that
show inner spiral arms composed by discrete structures of gas with a well defined inter-arm separation.
Key words. Galaxy: disk – Open clusters and association: general– Open clusters and association: individual: Trumpler 27 - -Stars:
early type – Galaxy: structure
1. Introduction
One of the authors (Carraro 2011) has very recently examined
the parameters of the young diuse stellar populations in the
direction l = 314
as seen in the background of some open
clusters and related them successfully with the inner galactic
structure. This study made use of photometric techniques that
we developed in the last decade and successfully applied to the
third quadrant of the Milky Way (Carraro et al. 2005; Moitinho
et al. 2006; V´azquez et al 2008).
In Carraro (2011), hereafter the first paper of the series, we
extensively described the project and the employed methods and
proceeded to study the spatial distribution of early type stars
Based on observations collected at Las Campanas Observatory,
Chile
⋆⋆
Full photometric data and Tables 2 and 3 are available at the
CDS via anomymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via
http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/999/A999
(spectral types O and B) in the direction of l = 314
. Apart from
the prominent Carina-Sagittarius spiral arm, we detected for the
first time signatures of the Scutum-Crux arm in that direction.
These findings confirmed the predictions of earlier studies on
the location and pitch angles of part of the spiral arm pattern in
the fourth Galactic quadrant.
The present article, the second of the series, is aimed at improv-
ing our knowledge of the spiral structure in the inner galactic
disk in another direction, much closer to the Galaxy center, at
l = 355
. In this case, instead of a single pointing as in the first
paper, we combined together three pointings to cover a wide
region ( 25 arcmin on a side) in the direction of Trumpler
27. This is an open cluster located in the Sagittarius constel-
lation, 5 degrees from the center of the Galaxy and slightly
below the galactic plane, at coordinates α
2000
= 17h36m20s,
δ
2000
= 33
31
(l = 355
.064, b = 0
.742). The zone
occupied by the cluster is heavily obscured by dust clouds but
nevertheless it is still possible to see a significant but sparse
1
Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk
group of relatively bright stars. The first extensive photometric
survey on this object was conducted by The and Stokes (1970)
who obtained UBV photographic photometry for about 40 stars
spread over a circular area of 11
radius around the assumed
center of this cluster. They found huge values for the color
excess E
(BV)
ranging from -approximately- 0.6 to 2.5 mag.
Despite this, they found that the extinction law is normal with
R
V
=
A
V
E
BV
= 3.1 and placed the cluster at a heliocentric distance
of 1.1 kpc. Later on, Moat et al. (1977, hereinafter MFJ77)
conducted a deeper UBV photometric survey including pho-
toelectric observations of the brightest stars and photographic
ones for the rest of them. This study was combined with
spectroscopic analysis for a significant number of stars (about
50) and suggested that Trumpler 27 is at a distance of 2.1 ± 0.2
kpc from the Sun with almost similar reddening values to the
ones previously computed by The and Stokes (1970). Bakker
and The (1983), used Walraven photometric data combined with
infrared and UVB photometry. This last study confirmed that
the extinction law is normal, i.e. R
V
= 3.1 3.2, that the cluster
is located at a distance of 1.7 ± 0.25 kpc and is about 10
7
years
old.
After a 20 years hiatus with no investigations, Trumpler 27
was the target of a new spectroscopic campaign carried out by
Massey et al (2001, hereinafter MDEW01) where the oldest
spectral types were verified and spectroscopy was made for
several stars that had not previously been measured. This new
spectroscopic survey revealed a surprisingly hot and massive
stellar population in the area of this object: one M supergiant,
10 supergiants of B-types, two Wolf-Rayet stars, one Of-type
star, two O-type giant stars, other stars of B-types and G0I
-probably- Cepheid star. In MDEW01 the parameters of several
stars in Trumpler 27 were redetermined onto the basis of this
new spectral survey but retaining the old UBV photometry
made by MFJ77. In this investigation the authors found that
the cluster is located at a distance larger than any previous
determination: 2.5 kpc from the Sun. The next interesting
finding made by MDEW01 is the detection of a significant
age spread amongst the B-type stars. It appears that some of
them show ages ranging from 6 to 10 × 10
6
years being about
2 to 4 × 10
6
older than the rest of them. They assume this
age spread comes from the large diculty in separating clus-
ter members from backgroundstars in this highly obscured zone.
Given its position -close to the Galaxy center and onto the galac-
tic plane- Trumpler 27 is an ideal target for the purpose of inves-
tigating part of the spiral structure in the fourth Galactic quad-
rant. Its surroundings contain a large number of potential blue
stars, some of which are considered excellent tracers of spiral
arms such as W-R stars and O-type stars as shown by MDEW01
and MFJ77. It is useful to recall that it has been almost 30 years
since the last photometric study on this object and that we are
now in the position of taking advantage of the large number of
spectral types to re-discuss the distance and age of Trumpler 27
with modern UBVI observations. These two parameters are of
paramount importance because this cluster may become a good
galactic spiral arm tracer if the distance discrepancy of the order
of about 1.4 kpc from one author to other is removed. In other
words, such a dierencemeans that the cluster maybelong to the
Carina-Sagittarius arm or the Scutum-Centaurus arm. But, first,
we should assess whether Trumpler 27 really exists and if the
found distance discrepancies are only due to the treatment of the
strong reddening of the region and/or to the fact that in clusters
so heavily obscured, like this one, only the upper -vertical- main
sequence is useful for fitting a reference line (e.g., the ZAMS of
Scmidt-Kaler (1982)).
In Section 2 we discuss the data acquisition, the reduction
process, the stellar astrometry and the comparison with former
photometry in the region. In Section 3 we show and discuss the
main photometric diagrams and introduce the strategy to derive
intrinsic parameters of stars in Trumpler 27. In Section 4, we an-
alyze the data set derived for hot stars and the natureof Trumpler
27. Section 5 includes a description of findings using infrared
data. In Section 6 we collect the evidence to interpret the struc-
ture of the inner Galaxy as seen through the Trumpler 27 region.
Conclusions are given in Section 7.
2. Observation and data reduction
Three regions containing Trumpler 27 were observed in the
UBVI photometric system at Las Campanas Observatory (LCO)
on four dierent runs in 2006, 2010, and 2011, as illustrated in
Table 1, which lists useful details of the observations, like filter
coverage, airmass range and exposure time and sequences. We
used the SITe#3 CCD detector onboard the Swope 1.0m tele-
scope
1
. With a pixel scale of 0.435 arcsec/pixel, this CCD al-
lowed us to cover 14.8 × 22.8 arcmin on sky. In all runs the
seeing was good, ranging from 0.9 arcsec to 1.8. However, post-
processing indicated that in the 2006 and 2010 runs, the nights
were not completely photometric, and for this reason we only
rely on the 2011 standard stars. The whole field of view is shown
in Fig 1, a montage of all V exposure images, and covers an area
of 26 × 23 squared arcmin. North is up, and East to the left.
To determine the transformation from our instrumental system
to the standard Johnson-Kron-Cousinssystem, and to correct for
extinction, we observed stars in Landolt’s areas PG 1323, PG
1633, and MarkA (Landolt 1992) multiple times and with dier-
ent air-masses ranging from 1.07 to 2.0, and covering quite
a large color range -0.3 (B V) 1.7 mag. All the images
from previous runs have been then shifted to the 2011 run by
means of all common stars, before transforming the photometry
into the standard system.
2.1. Basic photometric reduction
Basic calibration of the CCD frames was done using IRAF
2
package CCDRED. For this purpose, zero exposure frames
and twilight sky flats were taken every night. All frames were
pre-reduced applying trimming, bias and flat-field correction.
Before flat-fielding, all frames were corrected for linearity,
following the recipe discussed in Hamuy et al. (2006).
Photometry was then performed using the IRAF
DAOPHOT/ALLSTAR and PHOTCAL packages. Instrumental
magnitudes were extracted following the point-spread function
(PSF) method (Stetson 1987). A quadratic, spatially variable,
master PSF (PENNY function) was adopted, because of the
large field of view of the detector. Aperture corrections were
then determined making aperture photometry of a suitable
number (typically 15 to 20) of bright, isolated, stars in the
field. These corrections were found to vary from 0.160 to 0.290
1
http://www.lco.cl/telescopes-information/henrietta-swope/
2
IRAF is distributed by the National Optical Astronomy
Observatory, which is operated by the Association of Universities
for Research in Astronomy, Inc., under cooperative agreement with the
National Science Foundation.
2
Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk
Table 1. UBVI photometric observations of Trumpler 27 and standard stars.
Target RA DEC Date Filter Exposure (sec) airmass (X)
Field-1 17:36:20.79 -33:28:02.8 2006 May 31 U 20, 200, 2x1500 1.021.05
B 10, 100, 2x1200 1.001.00
V 5, 10, 100, 900 1.141.16
I
5, 10, 2x100 1.081.10
Field-1 17:36:20.79 -33:28:02.8 2006 Jun 26 B 40 1.001.01
V 20 1.001.01
I
20 1.001.01
Field-1 17:36:20.79 -33:28:02.8 2006 Jun 31 U 60 1.001.01
B 30 1.001.01
V 30 1.001.01
I
30 1.001.01
Field-1 17:36:20.79 -33:28:02.8 2010 May 08 U 15, 90, 300, 1500 1.021.03
B 5, 45, 300, 1200 1.061.10
V 2x5, 120, 900 1.101.15
I
2x5, 30, 600 1.011.01
Field-2 17:36:54.79 -33:28:02.8 2010 May 08 U 15, 90, 300, 1500 1.011.01
B 5, 45, 300, 1200 1.031.04
V 2x5, 120, 900 1.121.14
I
2x5, 30, 600 1.051.08
Field-3 17:35:47.79 -33:28:02.8 2010 May 08 U 15, 90, 300, 1500 1.051.11
B 5, 45, 300, 1200 1.111.18
V 2x5, 120, 900 1.361.42
I
2x5, 30, 600 1.201.28
Field-1 17:36:20.79 -33:28:02.8 2011 Jun 03 U 60, 180 1.001.01
B 30, 120 1.001.01
V 10, 60 1.001.01
I
10, 60 1.001.01
Mark A 20:43:55.09 -10:45:38.0 2011 Jun 03 U 180, 2x240 1.071.60
B 120, 2x180 1.071.56
V 60, 2x90 1.071.53
I
60, 2x90 1.061.51
PG 1323 13:25:38.91 -08:50:05.6 2011 Jun 03 U 3x180 1.081.73
B 3x120 1.081.77
V 3x60 1.071.82
I
3x60 1.071.85
PG 1633 16:35:24.91 09:47:39.8 2011 Jun 03 U 2x180, 240 1.291.83
B 2x120, 180 1.291.88
V 3x60, 90 1.292.02
I
2x60, 90 1.291.97
mag, depending on the filter. The PSF photometry was finally
aperture corrected, filter by filter.
2.2. Photometric calibration
After removing problematic stars, and stars having only a few
observations in Landolt’s (1992) catalog, our photometric solu-
tion for the run was extracted combining measures from both
nights- after checking that they were stable and similar- yielding
a grand total of 63 measurements per filter, and turned out to be:
U = u+ (5.004±0.010)+ (0.49±0.010)×X+ (0.129±0.013)×
(U B)
B = b+ (3.283 ±0.006)+ (0.25 ±0.010)× X+ (0.040 ±0.008)×
(B V)
V = v+ (3.204±0.005) + (0. 16±0.010)×X(0.066±0.008) ×
(B V)
I = i+(3.508±0.005)+(0.08±0.010)×X+(0.037±0.006)×(VI)
where X indicates the airmass.
The final r.m.s of the fitting in this case was 0.040, 0.019, 0.015,
and 0.015 in U, B, V and I, respectively.
Global photometric errors were estimated using the scheme de-
veloped by Patat & Carraro (2001, Appendix A1), which takes
into account the errors resulting from the PSF fitting procedure
(i.e., from ALLSTAR), and the calibration errors (corresponding
to the zero point, color terms, and extinction errors). In Fig. 2 we
present our global photometric errors in V, (BV), (U B), and
(V I) plotted as a function of V magnitude. Quick inspection
shows that stars brighter than V 20 mag have errors lower than
0.05 mag in magnitude and lower than 0.10 mag in (B V)
and (V I). Higher errors, as expected, are seen in (U B). The
final catalogue contains 9769 UBVI entries.
2.3. Astrometry
The optical catalogue was cross-correlated with 2MASS, which
resulted in a final catalog including UBVI and JHK
s
magni-
tudes. As a by-product, pixel (i.e., detector) coordinates were
converted to RA and DEC for J2000.0 equinox, thus providing
2MASS-based astrometry, useful for e.g. spectroscopic follow-
up.The rms of the residuals in the positions were 0.
′′
15, which is
about the astrometric precision of the 2MASS catalogue (0.
′′
12,
Skrutskie et al. 2006 ).
3
Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk
δ (2000)
α (2000)
-33.65
-33.60
-33.55
-33.50
-33.45
-33.40
-33.35
-33.30
17.59017.59517.60017.60517.61017.61517.62017.625
Fig.1. Montage of all V images (28) taken in the area of
Trumpler 27. The field is about 26 arcmin × 23 arcmin. α and
δ coordinates are given in decimal notation. North is up, East to
the left.
Fig.2. Trend of global photometric errors in magnitude and col-
ors as a function of our V magnitude. See text for details.
2.4. Comparison with former photometry
Up to now, the most extensive catalogue of UBV photometry
in the region of Trumpler 27 belongs to MFJ77. The other pho-
tometric sets are only photographic or in the Walraven system.
Comparing our data set with MFJ77 we found 83 common stars
in V, 81 in (BV) and 68 in (UB). The dierences in the sense
of our data minus MFJ77, are shown in Fig. 3 as a function of
our V. Most of the stars in the MFJ77 survey have photographic
photometry and just a few have photoelectric photometry. In the
comparisonprocess, we found at least half a dozenstars showing
strong dierences in apparent magnitude and/or color indices.
Fig.3. Dierences of our photometry with MFJ77 plotted
against our V magnitudes. Upper panel, δ
UB
, mid panel δ
BV
,
lower panel δ
V
.
Fig.4. The CCD -upper left panel- and the CMDs for (B V)
-upper right panel-, for (U B) lower left panel and for (V I)
-lower right panel of all observed stars. Solid line in the CCD
is the Schmidt-Kaler (1982) intrinsic line. Dashed lines in the
CCD mean the path of the reddening for a B5- and an A0-type
stars as indicated by the labels. The bluish region represents the
location of nearby dwarf stars; the yellowish one points out the
locus occupied by A-F-type stars aected by increasing redden-
ing. Stars above the B5 reddening line are young stars (see text),
except those ones for which no reddening solution is possible.
4
Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk
This may be produced, specially in crowded regions, in an un-
avoidable star-lightcontaminationby nearby neighborswhen us-
ing diaphragms in photoelectric measures. Mean dierences and
standard deviations are
δ
V
= 0.08±0.15,δ
(BV)
= 0.19±0.15
and
δ
(UB)
= 0.23 ± 0.17. The systematic trend in Fig. 3, in
the sense that our values are brighter and bluer than those of
MFJ77 and the large scatter, is likely produced by some zero
point dierences between our photometry and MFJ77’s photo-
graphic and photoelectric photometry made with dierent tele-
scopes and dierent diaphragm apertures, respectively.
3. The all area photometric diagrams
Earlier CCDs and CMDs of the region of Trumpler 27 have been
always very dicult to interpret because of strong and variable
internal reddening and the consequent diculties in defining the
upper main sequence. Besides, the upper part of Trumpler 27
CMD is quite vertical and short what makes any fitting of the
ZAMS (Schmidt-Kaler 1982) dubious. The sum of these di-
culties have precluded this object from a more in-depth study so
far (McSwain & Giess, 2005).
Figure 4 shows the CCD (upper left panel), the V vs (B
V) (upper right panel), the V vs (U B) (lower left panel) and
the V vs (V I) (lower right panel) CMDs. The solid line in
the CCD is the ZAMS of Schmidt-Kaler (1982) shown in the
normal R
V
= 3.1 position. The reddening path for a B5-type star
is also indicated. A simple visual inspection of the CCD allows
to establish three dominant stellar populations:
a first one, above the reddening line for a typical B5-type
star, composed undoubtedly of blue stars strongly aected
by variable reddening;
a second stellar population appears below the reference line
for a B5 star (the yellowish zone) composing a sort of stellar
band starting at the second knee of the ZAMS and extending
to the red side because of dierent amounts of reddening af-
fecting them. This large star concentration corresponds,pho-
tometrically speaking, to A and early F-type stars but also to
reddened B-type stars later than B5 as indicated by the path
of the reddening in the CCD;
finally, a third stellar population (greenish coloured) little af-
fected by reddening appears extending from the domain of
A to M stars all along the intrinsic reference line. Most of
these stars are likely to be nearby objects since visual ab-
sorption maps from Neckel and Klare (1980) reveal a sud-
den absorption increment at less than 1 kpc in this direction.
Therefore, the detection of intrinsically faint stars not red-
dened or just marginally aected by interstellar dust is only
possible if they are close enough to the Sun.
Inspection of Fig. 1 lends further support to this scenario:
intense absorptionis seen at the west-southwest andimmediately
north of the center of Trumpler 27, and also to the east. A clear
star overdensity is seen, in the same figure, at the northeast and
at the expected location of the cluster core.
The CMDs [V vs (BV), V vs (U B) and V vs (V I)] are
less illustrative since not much can be extracted from them be-
yond the presence, aboveV = 16 mag, of an extended upper stel-
lar sequence with a high degree of color dispersion that reflects
the strong influence of dust in the zone. Since the three popula-
tions discussed previously are undistinguishable from each other
in the CMDs, we estimate this type of diagram plays a marginal
role to inferring useful information on this region.
3.1. Deriving the parameters of early-type stars
If the CMD is segmented with magnitude intervals and the cor-
responding CCD to each interval is built, it becomes simpler
to understand the way stellar populations develop in a given
zone. The foundation and applications of this kind of analysis
in open clusters and Galactic fields is developed in Carraro et
al. (2008, 2009). In the present case the procedure is shown
in the series of CCD in Fig. 5 at intervals of 1 mag down to
V = 18. The superimposed lines follow the slope E
(UB)
=
0.72×E
(BV)
+ 0.05×E
2
(BV)
explained long time ago by Hiltner
and Johnson (1956) (correspondingto R
V
= 3.1) for stars of var-
ious spectral types from O to B5 in addition to the A-type stars.
The series of panels shows the color range covered by the
three above mentioned star populations. Actually, reddened hot
stars along the reddening lines for O and B4 types look well
detached from the rest down to V 14. A comparison of our
CCD with the ones produced by Bakker and The (1983), in their
Fig. 3, or in Fig. 5 of MFJ77, suggests that the major dierence,
apart from the inclusion of many faint stars, rests in the high
number of newhot and reddenedstars detected in our fieldwhich
in turn is larger than the others.
The panel in the right lower corner shows the overall CCD
where we have identified, with open squares, bright stars already
studied in previous articles via spectral analysis.
From V 14 a stellar group (yellowish coloured) composed
of a wide strip of A and F-type stars becomes evident reaching
the maximum at V = 15. Certainly, this latter group shows an
ample range of reddening. Beyond this limit dwarf stars of G-K-
and even some M-type stars are detected at increasing number all
along the intrinsic line indicating they are not so much aected
by reddening and belong to the greenish zone. As stated above,
these stars and part of the less reddened A-F- stars in the group
belong to a nearby population in the line of sight to Trumpler 27.
In the remaining of this paper we shall focus on those stars
above the reddening line for B5-type stars as indicated in Fig.
5 to derive individual distances by means of the spectroscopic
parallax method since we know they have unambiguous red-
dening solution in the CCD. This method has proved to be
very ecient to approximate spectral types from photometry
alone when unique reddening solutions are available as shown
in Ahumada et al. (2011). Intrinsic colors for each star in this
zone of the CCD were derived using the reddening line slope
E
(UB)
= 0.72 × E
(BV)
+ 0.05 × E
2
(BV)
along with the relation
(U B)
0
= 3.69 × (B V)
0
+ 0.02 (Carraro et al. 2010). This
gives a second order equation where the (B V)
0
index is the
positive root and the (U B)
0
index comes from the above ex-
pression. This locates the star on a point in the ZAMS (Schmidt-
Kaler 1982) where the nearest spectral type is assigned. Unlike
the Q-method that derives the (B V)
0
star color assuming a
linear relation of color excesses, here we adopt a quadratic rela-
tion that provides a more accurate solution when the color excess
is high. Once individual color excesses and intrinsic colors are
derived,we proceed to assign spectral types and absolute magni-
tudes following the relationship given by Schmidt-Kaler (1982).
In this procedure all stars are assumed to be of luminosity class
V. However, a good number of hot stars have spectral types and
luminosity class from MDEW01 and MFJ77. In these cases, the
assignment of intrinsic color and absolute magnitude depend on
the spectral type and luminosity class found by these authors us-
ing again the Schmidt-Kaler (1982) calibrations. Individual dis-
tance moduli are given after correcting apparent magnitudes by
A
V
= 3.1×E
(BV)
. We accept R = 3.1,which is completelyin line
with the rigorous study of the absorption throughout Walraven
5
Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk
Fig.5. The CCDs of the segmented CMD at intervalsof 1 mag. Dashed line shows the position of the Schmidt-Kaler (1982) intrinsic
line. The succession of solid lines show the path of the reddening for some of the early type stars indicated with the respective labels.
The panel in the right lower corner shows the overall stars and those with spectral types, grey squares, from MFJ77
Table 2. Intrinsic photometric values, distances and spectral types of stars in Trumpler 27
Id E
(BV)
(B V)
0
(U B)
0
M
V
d σ
d
ST
1
Id(MFJ77) ST
2
(MFJ77,MDEW01)
5517 1.69 -0.28 -1.00 -2.60 3.17 0.73 B0.5 (...) (...)
4732 1.10 -0.28 -1.00 -2.60 2.64 0.58 B0.5 19 OB-
5355 1.14 -0.21 -1.02 -6.90 6.60 0.29 B0.5Ia 16 B0.5Ia
6456 1.12 -0.02 -0.62 -7.10 6.74 0.15 B0Ia 2 B0Ia
5672 1.80 -0.27 -0.95 -2.35 3.38 0.78 B1 (...) (...)
Id first column is our star numbering. Id column nine is the MFJ77 numbering. ST
1
indicates the photometric spectral type
found in this paper. ST
2
(MFJ77,MDEW01) indicates spectral types from the literature.
Table 3. Idem for stars outside Trumpler 27
Id E
(BV)
(B V)
0
(U B)
0
M
V
d σ
d
ST
1
Id(MFJ77) ST
2
(MFJ77,MDEW01)
442 1.22 -0.25 -0.90 -2.10 3.41 0.84 B1.5 (...) (...)
4497 1.09 -0.25 -0.90 -2.80 2.83 1.33 B1.5:V: 32 B1.5:V:
3629 1.15 -0.26 -0.95 -3.20 3.22 0.95 B1:V: 34 B1:V:
8932 1.25 -0.26 -1.00 -5.40 2.64 0.40 B1II 103 B1II
Columns as in Table 2.
and infrared photometry developed by Bakker and The (1983)
in the region.
The distances obtained are photometric and are subject,
therefore, to uncertainties depending primarily on photometric
errors. As donein Carraro (2011),we analyze the impact of these
errors on distances by starting with the well known expression
for the distance modulus:
V M
V
= 5 + 5 × log(d) + A
V
(1)
6
Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk
Fig.6. The left panel is the contour plot showing the star density in the surveyed area generated with 9000 stars approximately.
The insert gives the number of stars per half-minute squared box. The right panel shows the contour plot of those stars early than
B5-type (see text). The insert as in the left panel. The 600 pxl circle indicates the region assumed to be the core of the cluster.
Fig.7. The left panel stands for the spatial distribution of early type stars according to their spectral types -shown in the insert- as
explained in the text. The right panel is the contour plot of E
(BV)
across the cluster surface. The insert gives the amount of E
(BV)
.
The circle has the same meaning as in Fig. 6.
A simple error propagation leads to:
ǫ
(d)
= ln(10) × d × 0.2 ×[σ
V
+ 3.1 × σ
(BV)
] (2)
Assuming that ǫ
(A
V
)
= 3.1×ǫ
(BV)
, where σ
(V)
and σ
(BV)
directly
come from photometry. In the final error computation we adopt
ǫ
(M
V
)
= 0.
The results of this procedure yielded about 600 stars with
intrinsic parameters and distances as indicated in Tables 2 and
3 in a self explanatory format. Inspecting the errors in distance
in the same tables, it follows that typical errors are about 20%,
quite reasonable for the method and of the same order of the
obtained by the usual ZAMS (e.g. Schmidt-Kaler 1982) super-
position. We want to emphasize that the assignment of spectral
types entirely rests on the star positions onto the CCD (Carraro
2011) as if they were single stars. We recall that some of them
may be undetected binaries or unresolved double stars what in-
troduces another source of distance uncertainty due to altered
colors and magnitudes. Even if we deal with true single stars
having MK spectral types this is not enough to reduce uncer-
tainties in distances since the MK classification and therefore
the assigned M
V
may vary from author to author (see cases of
dierent classification in Tables 2 and 3). Finally, it is evident
7
Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk
that even amongst the stars located along the reddening line for
B5-type stars there is still a chance of contamination by intrinsic
data scatter of some reddened A-type stars. Therefore, and in an
attempt to minimize wrong star inclusions, we restrict the spec-
troscopic parallax method to those stars in the range from B4.5
to O-types (unless otherwise stated). We are confident that, de-
spite the errors, our large data sample will clearly revealthe main
propertiesof the stellarpopulations in the region of Trumpler 27.
3.2. Discussion on spectral types and distances of some
stars
Here we briefly discuss the nature and parameters of a few
particular stars listed in Tables 2 and 3 for which distances
have been derived using the spectroscopic parallax method.
We focus mainly on some stars for which MK spectral types
and thin-prism classification comes from the literature. We also
discuss briefly some potential variable stars. We refer to all
these stars using the identification given in MFJ77.
Star 1: MDEW01 confirm the oldest spectral type, M0Ia,
given by MFJ77. It is also a suspected variable (NSV 22849)
as indicated by Samus et al. (2010). The distance of 1.8 kpc
has been derived under the assumption that it is not a variable.
Additional information on this objects is commented later when
using infrared data.
Star 2 (LSS 4253): the old spectral type from MFJ77,
O9Ia, turns out to be B0Ia according to MDEW01 with no
indication about membership. This last spectral type and our
new photometry locate it at almost 7 kpc from the Sun, outside
any reasonable distance limit given for Trumpler 27 in the
literature.
Stars 8, 10, 12, 19, 20, 21, 25 and 30, all have thin-prism plates
classification in MFJ77 (a classification technique that indicates
whether a star is a potential OB star) and none of them were
studied spectroscopically by MDEW01. In most of these cases
they were classified as potential OB stars with dierent degrees
of accuracy as seen in Tables 2 and 3. Our procedure confirms
the MFJ77 thin-prism plates classification approximation as
they resulted of photometric spectral types O4, O7, O8.5, B0.5,
B1.5, B1, O4 and O6-types, respectively.
Star 16 (LSS 4263): the early spectroscopy gave O9.5 II: but
changed to B0.5 Ia after MDEW01. The new spectral type and
our photometry locate it at a distance of about 7 kpc.
Star 23: The spectral type determined by MFJ77, B0.5Ia, was
changed to B0.7Ia after MDEW01. This star is also known as
V1082 (Samus et al. 2010), a probable eclipsing variable. We
derived a distance of 3.5 kpc but if it becomes a true binary star,
the distance could be reduced to 2.6 kpc in the most favorable
case of similar masses.
Star 44: this star is a B1.5Ia according to MDEW01 who
have given concluding spectroscopic arguments favoring it is a
supergiant star and not a giant one as stated in MFJ77. In view
of the distance of 2.5 kpc they computed for Trumpler 27, they
assumed this star is not a cluster member, an assertion we agree
with because the distance we obtain with our photometry is
about 16 kpc from the Sun. Even if we assume it is a binary star
its distance would still be more than 10 kpc away from the Sun.
The WR stars: on one side, star 28 (WR 95) was classified
WN5 by MFJ77 and WC9 by MDEW01; we adopted here this
latest and modern classification. On the other side, star 105
(HDE 318016) is a WC7/ WN6 following MDEW01. Conti and
Vacca (1990) reported distances of 2.8 kpc for the WC9 and 2.4
kpc for the WC7/WN6 that were adopted by MDEW01 in their
analysis. To deal with these two complex stars we proceeded to
derive their intrinsic (B V) colors by means of the relationship
between the Smith (1968) ubv and UBV systems as given in
Lundstr¨om & Stenholm (1984). According to the relationship
given in this last paper, it was assumed (b v) = 0.4 for the
WC9 (WR 95) and (bv) = 0.3 for the WC7/WN6 (105) while
the absolute magnitudes were taken from the newest values
given by van der Hucht (2001). Excesses of color obtained using
the relationships of Smith (1968) were E
(BV)
= 1.95 for WR
95 and 1.4 for star 105. Finally we derived distances of 2.2 for
WR95 and 3.5 kpc for star 105 that we will use in the analysis
next section.
Star 107 (LSS 4257): it was classified B0V by MFJ77 but the
new spectral type from MDEW01 drastically changes the point
by setting this star as a supergiant of a later type, B0.5Ia. As
in the previous case of star 44, they concluded this is another
non-member star. We agree since our distance sets this star at
more than 10 kpc from the Sun. Even in case of binarity, its
distance would be reduced to 7.6 kpc, too far from any previous
estimate of the cluster distance.
Star 102 (V925 Sco, HD159378): this is a variable star with
no well established period from 70
d
to more than 300
d
and
light variation of a few tenths of a magnitude (Samus et al.
2010). Since, according to van Genderen (1980), it may not be a
Cepheid variable of Pop I, we treated it as a G0 supergiant star
and derived a distance of 2.4 kpc not far from the 2.1 kpc given
by MFJ77. Parthasarathy & Reddy (1993) placed this object at 3
kpc from the Sun but they gave no precise indication on the way
they got their reported distance.
4. Hunting for Trumpler 27
We made an attempt to use proper motions in the area of
Trumpler 27 from the UCAC3 data-base (Zacharias et al. 2010).
The putative distance (at 2-3 kpc from the Sun) and the crowd-
ing of the region towards the Galactic center made the exercise
useless. Errors in both proper motion components are large and
no feature can be detected. This is exactly as in Moni Bidin et al.
(2011), where three possibly new globular clusters are discussed
toward the Galactic bulge using data from the VISTA VVV sur-
vey (Minniti et al. 2011). As in that case, proper motions are so
scattered that we even restrain from showing the results.
In previous papers we have explained (see e.g. V´azquez et al.
2010) that instead of using star density profiles to set the cluster
boundaries we prefer to look for the zone of higher star density
in a contour plot. Indeed, density profiles reduce an open cluster
-irregular in shape by definition- to a centrally peaked spherical
stellar density distribution. On the contrary contour plots reveal
details of the impact of absorption on the cluster shape and allow
to define the boundaries in a more realistic approach. The pro-
cedure works well in areas where the absorption is high like the
one in this paper. As in the case of a radial density profile we set
the size of a cluster to the area enclosed when the contour plot
reaches a flat value. Notwithstanding and for the sake of com-
pleteness of the present analysis, in the next section we shall ex-
amine the infrared density profile. Meanwhile, Fig. 6 (left panel)
8
Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk
shows the contour plot (0.5 × 0.5 arcmin box using the Kriging
gridding method) generated with more than 9000 stars detected
in the region of Trumpler 27. The full contour plot shows no
star overdensity at all in the place where the cluster is assumed
to be (the bright stars near the center of the mosaic in Fig. 1).
Overdensities are detected north and south of the location of the
cluster center confirming what a simple eye-inspection shows in
Fig. 1. Actually, the lack of a notorious overdensity right there
suggests that Trumpler 27 has been detected for being handful
of bright stars well detached from the rest by dust clouds. This is
better shown in the right panel of Fig. 6 which displays the con-
tour map generated only with all early-type stars of Trumpler
27 included in Tables 2 and 3 (spectral types down to B4.5). The
circle in this panel has 600 pxl radio, just over 4 arcmins and sur-
rounds entirely the weak overdensity emerging at the position of
the cluster. That is, the contour map in the right panel shows that
the cluster is just a handful of early stars separated by a thick
dust ring from other similar groups in the region. Figure 7, left
panel, displays stars in Tables 2 and 3 according to their spec-
tral types, describes clearly the above assertion. Note that the
stellar density in this region cannot be assumed dierent from
that north-west and west. The role played by the absorption is
also well depicted by the contour map of E
(BV)
excesses built
in the right panel of Fig. 7 with the values listed in the tables.
Here we see that reddening varies in the range from 1 to more
than 2 mag across the surface with higher values in a wide band
from the south- south-west to the north-east that separates the
diverse groups seen in the left panel. A comparison of the right
panel in Fig. 7 and the left panel (all stars) in Fig. 6 suggests that
a vast fraction of the A-type stars surveyed in the area may be
immersed in the dust and just a few of them together with oth-
ers of later types are likely located in front of the dust clouds.
Otherwise, the overdensities seen north and south-east in the left
upper panel should disappear in this plot.
The zone of Trumpler 27 was included by Bitran et al. (1997)
in a CO survey focused in a dozen degrees around the Galaxy
center. The cluster is not only immersed in a region of large
CO emission but, most interesting, it is projected against a zone
of extremely high absorption as shown in map 222 made by
Neckel and Klare (1980). This alone justifies the appearance of
our CCDs in Fig 5 where potential A-F-type stars start with lit-
tle absorption to end up, at V = 16, completely immersed in the
dust clouds. Likewise, the latest spectral type stars in the field
have to be necessarily nearby stars since they are intrinsically
too faint and do not appear much aected by reddening.
Figure 8 traces the path of reddening with distance inside
and outside the cluster boundaries. Error bars coming from Eq.
(2) are also shown. It is worth mentioning briefly the impact
of distance errors into our analysis. We discussed in previous
section the meaning of distance errors which are mainly based
on the propagation of photometric errors which are random in
nature. Fig. 8 shows the trend of our sample of stars toward
the Galaxy center and we do not see that errors can modify
any of our conclusions. When looking through the core of the
Trumpler 27 region the bulk of early type stars extends for more
than 4 kpc while some others may reach 9 kpc and even more
than 15 kpc as indicated. Something similar happens outside
the 600 pixels limit with some stars located at large distances
too. The reddening behavior shown in this figure is well in line
with the above mentioned absorption map 222 from Neckel and
Klare (1980). Map 222 and nearby ones show that absorption
increases after 0.5 kpc from the Sun, rises up to 5-6 mag in
the next kiloparsec and keeps high up to 4-5 kpcs towards the
Galaxy center. Our deeper and extended sample of young stars
Fig.8. The path of reddening with distance for stars inside (up-
per panel) and outside (lower panel) 600 pxl radius. Horizontal
bars are distance errors computed from (2). Some of the distant
stars are indicated. Numbers in parentheses mean the MFJ77 no-
tation.
not only confirms findings of Neckel and Klare (1980) but also
states strong absorption variability all across the region with
0.65 < E
(BV)
< 2.2 mag meaning more than 6 mag of visual
absorption. Fig. 8 shows too that dust clouds are not a local phe-
nomenon but they are presents for several kiloparsecs starting
near the Sun and spanning over 4 kpc (at least) towards the cen-
ter of the Galaxy.
Histograms for dierent spectral types as a function of dis-
tance for both regions are shown in Fig. 9. Left and right panels
are for stars inside and outside Trumpler 27, respectively. The
upper panels include O-. WR-, B0- and late type supergiants.
Mid panels is for B1-B2- type stars and lower panels is for B3-
B4- types. For the sake ofan easy visualization, we restricted the
plot to all the stars up to 7 kpc from the Sun.
In terms of confirmed spectral types, we found that 19 stars
out of 126 inside the cluster limits have some type of spec-
tral classification (MK or objective prism) from Massey and/or
MFJ77 including O- and B-type stars and one WC9 star; the
supergiant star No 1 belongs to this zone. Outside the cluster ra-
dius there are 501 early type stars; 9 of them, including the G0
Cepheid variable, have spectral analysis: one WC7/WN6, one
O8.5III and one early B-type supergiant.
Inside the cluster limits O-type stars are seen nearly at all
distances. The rest of stars show a narrower distribution beyond
1.5 kpc from the Sun. Late dwarf B-types are lacking in the first
1-1.5 kpc probably because of dust but also because of an statis-
tical artifact. The lack of them at larger than 4-5 kpc is mainly
due to a combinationof extinctionand intrinsic faintness of these
spectral types. In the left panels there seems to be two peaks, one
at 1-1.5 kpc and the other -higher- at 3.5 kpc. The 1-1.5 kpc
peak is not far from the estimated distances by The and Stokes
(1970) and Bakker & The (1983) for Trumpler 27 although it
is a bit far, however, from the 2.1 and 2.5 kpc given by MFJ77
and MDEW01, respectively. A more refined analysis indicates
9
Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk
Fig.9. Histograms of stars for spectral type. Left panels for those
inside the cluster limits and right panels for those stars outside
the cluster (see text for explanation).
this peak contains about 20 stars including three O-type stars at
1.5 kpc. and six other hot stars with distances between 0.9 and
1.6 kpc. The rest of the stars of photometric B-types (eight of
them have evolved MK types) spread over distances from 1 to
2 kpc. Three of them with confirmed evolved MK types are in
a distance range between 1.1 and 1.7 kpc. The M0Ia supergiant
star is placed at 1.8 kpc in our distance estimation but since col-
ors of extremely red stars may be quite uncertain it is likely lo-
cated at the same distance than the others. The WC9 star, placed
at 2.2 kpc, is unlike to be related with this star group. The mean
distance of all the stars in this stellar grouping (allowing for dis-
tance errors) is 1.7 ± 0.4 kpc. This is almost 1 kpc below the
value derived by MDEW01. Compared to other distances com-
puted for Trumpler 27, it is 0.5 kpc less than the one estimated
by MFJ77, 2.1 kpc, and about 0.5 kpc larger than another value
given by The & Stokes (1970), 1.1 kpc, but is in good agreement
with the distance published by Bakker & The (1983), 1.6 kpc
from the Sun. However, the simple grouping by itself -with huge
distance spread- does not indicate the presence of a true open
cluster as we shall see later on. If we look outside the region
of the location of Trumpler 27 in Table 3 we find only 35 stars
sharing a similar distance range.
Regarding the second star peak at 3.5 kpc seen in the di-
rection to Trumpler 27 core, it is quite far from the distance 2.1
and 2.5 kpc given for Trumpler 27 by MFJ77 and MDEW01.
Moreover, it is wider than the first one and tends to show some
kind of coincidence with the evident peak of stars outside the
cluster region at the same distance approximately (see right pan-
els in Fig. 9). In this distance range from 3 to 3.5 kpc there are
more than 110 stars outside the potential cluster core including
3 stars of O-types and one B-type supergiant out of 27 stars with
MK types. The WC7/N6 and the G0I stars, both at 2.4 kpc ap-
proximately, are probably not related with this second peak of
stars either.
The histograms ofFig. 9 are conclusiveinthe sense that early
type stars are found at any distance inside and outside the cluster
Fig.10. CMDs of stars ordered by distance. Symbols are: (+)
stars from 0 to 2 kpc, () for 2-3 kpc, () for 3-4 kpc, () for 4-5
kpc and () stars beyond 5 kpc. Upper panel, all stars aboveB5-
spectral types in Trumpler 27 region. Lower panel, idem above
for stars outside.
region. That is why,in our opinion, dierent authorsarriveat dif-
ferent conclusion about members and distances of this putative
open cluster. An additional proof is provided by the observed
CMDs of early type stars found inside and outside the cluster
boundariesshownin Fig. 10. Here stars havebeen identified with
dierent symbols according to their distances and immediately
arises that any combination of them may produce a believable
CMD. In particular, the stars in the 3-4 kpc range are responsible
for the left envelope and high density of both CMDs while the
most dispersed stars are those belonging to the 2-3 kpc range.
To be remarked is that inside the limits, where it is assumed
Trumpler 27 to exist, the upper part of the CMD is composed
by stars as near as 2 kpc and as far as more than 5 kpc from the
Sun.
10
Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk
From the above analysis we conclude then that the evidence
is not conclusive enough to clearly establish the true nature and
location of Trumpler 27. There is no doubt here that we are deal-
ing with a diuse young population of stars distributed all along
the line of sight with some stellar clumps including all types of
bright stars.
5. Infrared information
Infrared data provided by the GLIMPSEII spring’08 (highly re-
liable) catalogue was cross-correlated with our optical catalogue
to get 2MASS JHKs and IRAC 3.6, 4.5 and 8 µm data. In par-
ticular, JHKs data was used to perform an additional search for
Trumpler 27 as can be seen in Fig. 11 (down to J = 16 mag).
Using all stars in the GLIMPSEII catalogue which lie inside the
boundaries of our observed frames, a radial density profile was
constructed as shown in the upper left panel of Fig. 11. The pro-
file gives the number of stars per square arcmin computed as the
number of stars found in concentric annuli 1 arcmin wide start-
ing at the current adopted cluster coordinates given in §1. Error
bars are Poisson errors (computed as
N with N the number
of stars per square arcmin). The panel confirms that the cluster
is only an artifact produced by the evident dust ring around the
center as shown by the pronounced dip at 3 arcmin from the cen-
ter. This is the same shown in Fig. 6, left panel. Apart from the
dip the star density profile is essentially plain, within the Poisson
errors, showing no evidence of a star clustering.
Close to star No 1 and 1b (MDEW01 numbering) there ex-
ists the maser source OH355.1-0.76 measured by Knapp et al.
(1989) in their circumstellar CO emission survey. Unfortunately
they give no further information on this object since and it was
discarded due to the strong contamination caused by galactic CO
emission over a wide range of velocity. We assume that, proba-
bly, the supergiant star No 1 is the optical counterpart of this ob-
ject since that, in turn, is also a suspected variable (Samus et al.
2010). Across the area surveyed in this article we found optical
counterparts of two YSO candidates (Nos 5338 and 5269 in our
numbering, corresponding to 2MASSJ17362441-3331289 and
2MASSJ17362515-3325521 sources, respectively) and another
star (No 1796, our numbering) with huge infrared excess.
Upper right panel in Fig 11 is the overall (J H), (H
K) two color diagram. The intrinsic relation from Koornneef
(1983), solid black line, and the 6 mag reddening vectors
E
(JH)
/E
(HK)
= 1. 7, gray dashed lines, for early and late type
stars are indicated. We have superimposed stars earlier than B5
inside 600 pxls radius (white squares) and outside these limits
(red squares). Despite the fact that data scatter is strong and typ-
ical infrared errors are large, this diagram confirms that stars in
the region of Trumpler 27 are aected by strong variable redden-
ing and shows no indication of a clear cluster sequence. Indeed,
the spread of the data inside 600 pxl is similar to the one shown
by the other early stars outside this limit. Large blue squares and
the arrow show the position of the YSO candidates above men-
tioned and the star No 1796. It seems that more objects in the
zone exhibit large infrared excesses. The other two lower pan-
els are the K, (J K) and V, (J K) color-magnitude diagrams.
These two diagrams show the same pattern found in Fig. 4 in the
sense that no clear cluster sequence emerges from them and no
evidence of a pre-main sequence is revealed. The illusory verti-
cal strips seen in the K, (J K) are composed by stars at very
dierent distances inside the 600 pxl radius and also outside that
limit. Actually, with no other information available to impose
boundary conditions (as the one provided by the CCD in Fig.
4 and MK types) it would be possible to confuse the vertical
Fig.11. The radial density profile in the region of Trumpler 27
-upper left panel. Bars are Poisson errors. The (J H), (H
K) color-color diagram of all stars in the region is shown in the
upper right panel. White and red squares are stars in Tables 2
and 3 respectively. The cross indicates the average color errors.
Large blue squares give the position of the YSO candidates and
the arrow points out the position of the star No 1796 outside
the diagram limits. Solid black line is the intrinsic relation from
Koornneef (1983); the two grey dashed lines are color excesses
relations for A
V
= 6 mag for early and late type stars. The lower
left and right panels show the K, (J K) and V, (J K) color-
magnitude diagrams. Symbols as in the upper right panel.
strip with the upper sequence of an unreal cluster. For the sake
of completeness we have also plotted 3.6, 4.5 and 8 µ data in
the adequate diagrams (not shown to save space) and, again, no
evidence for a cluster has been found in any of them.
6. The region toward l = 355
and the inner spiral
structure of the Galaxy
The presence of over 600 stars of very early spectral types in a
small sky area of a quarter of square degree located in a contin-
uous range of distance up to 7 kpc at least from the Sun is an ex-
citing result of our survey that connects this young diuse pop-
ulation to the inner Galactic structure. In the traditional model
of four arms for the galaxy (Russeil 2003, Vall´ee 2005) mov-
ing from the Sun location inwards -in the region of Trumpler
27- there should be Sagittarius-Carina arm, Scutum-Crux arm,
Norma and the Near 3 kpc-arms at 0.6, 3, 4 and 5 kpc re-
spectively. In the proposed recent view face-on of our galaxy
(Churchwell et al. 2009), the cross with Sagittarius-Carina arm
should happen at 1.6 kpc. In particular, this arm looks weak and
narrow; this is, the arm is tiny and not very prominent. In the
11
Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk
same sketch the Scutum-Crux arm is located at 3.2 kpc and be-
comes the most prominent feature of the inner galactic structure.
The Norma arm is another tiny structure intersected at 4.4 kpc
and, finally, the Near 3 kpc arm at over 5 kpc is the less pro-
nounced of all the features. A recent map of the Milky Way has
been presented by L´epine et al. (2010) where the traditional four
arm structure is abandoned and replaced by polygonal arms de-
scribed by molecular CS associated with IRAS sources. Given
the complex structure proposed by L´epine et al. (2010) we pre-
fer to compare our findings with the traditional description (e.g.
Russeil 2003).
In terms of the stellar components, mainly O-B3-type stars
and open clusters associated with the grand design spiral struc-
ture, the information is scarce as shown by Russeil (2003). The
OB type stars catalogue recently compiled by Reed (2003) has
been used by Carraro (2011) to highlight the spiral structure
around the Sun. Fig.1 in Carraro (2011) demonstrates that only
O-type stars show some sort of clear coincidence with spiral
arms of the Galaxy while B-type stars, on the contrary, show a
smooth distribution up to the reach of the survey made by Reed
(2003). Carraro (2011) also emphasizes the lack of coincidence
of the star sample (O- plus B-types) with the trace of Scutum-
Crux. In the direction we are dealing with, the Reed (2003) cat-
alogue scarcely reaches 5 kpc from the Sun while the number
of confirmed(spectroscopically)O-type stars byMa´ız-Apell´aniz
et al. (2004) -complete only to V = 8 mag- indicates that very
few confirmed stars of this type are found in the direction where
Trumpler 27 is located.
Returning to our data sample we have included stars of
Tables 2 and 3 in the distance vs E
(BV)
plot of Fig. 12 up to
18 kpc from the Sun. In the figure we also have marked the dis-
tances to which the spiral arms of the Galaxy should be located
approximately according to the current accepted model of four
arms and their respective widths. We briefly recall that while
arms separation is of the order of 3 kpc the width of them is
quite uncertain. We set arms widths in Fig. 12 using the value of
about 1 kpc given by Vall´ee (2005). Accepting that Sagittarius-
Carina is at less than 1 kpc in the 355
direction we found very
few stars associated with this spiral arm, mostly O and B3-B4,
while the bulk of young stars tend to gather at the distance where
Scutum-Crux and Norma arms are set. The plots in Fig. 12 are
in line with Fig. 1 shown by Carraro (2011) in the sense that
very young stars do not define conclusively the trace of inner
spiral arms but are distributed evenly across several kiloparsecs
toward the center of the galaxy. This is a singular dierence with
radio observations. In fact, maps of the HI and CO contributions
suggest that these arms are discrete and well outlined structures,
Scutum-Crux being one of the major of them moving toward the
inner galaxy.
The recent study by Dame & Thaddeus (2008) collected spe-
cific and irrefutable evidence of the presence of the distant coun-
terpart of the Near 3 kpc arm based in CO and previous obser-
vations in HI. The authors estimated that distances from the Sun
for both arms are 5.1 and 11.8 kpc for the Near 3kpc arm and
the Far 3 kpc arm, respectively. It is worth mentioning that stars
at all spectral types (also one O-type) can be found in or close
to the Near 3 kpc arm in Fig. 12. If the tendency in our sample
is correct, for the first time we are looking at components of a
very recent star formation process there. From the point of view
of our findings the relation between the young population and
the Near 3 kpc arm is not controversial. As Dame & Thaddeus
(2008)show, the Near 3 kpc arm extendsmostly belowthe galac-
tic plane at the location of Trumpler 27 and therefore, part of the
young stars in our sample may be spatially connected with this
arm. There are stars even farther in this region (at more than 8
kpc) that have no chance to be related with the Far 3 kpc arm
since it extends mainly above the galactic plane.
From our study, based on an extended sample of early type
stars detected photometrically,we find it hard to establish the re-
lation of the inner spiral arms with our stellar population. While
the gas component is expected to trace the path of every inner
arm by means of discrete structures of molecular CO and HI,
stellar distances of young stars do not. Fig. 12 allows us to as-
sure that the trend of the young diuse population extends all
along 7 kpc towards the center of the Galaxy but by no means
can we specify the position and distance from the Sun of each
arm. This diculty arises from a combination of distance un-
certainties with geometrical circumstances. In fact, despite the
process of star formation is active and vigorous it is hard to re-
late it to specific patterns of spiral structure because of a) the
tight packing of four arms in such a short distance range and b)
our line of sight on the plane is almost at right angles with the
spiral structure.
All the circumstances mentioned above are reflected in the
radio observation domain. Very recently, Green et al. (2011)
have been studying the distribution of methanol masers in the
inner galaxy covering the longitude range from -28
to +28
in
longitude. In particular, in the direction of our analysis, they
found an enhancement of methanol sources which are com-
prehensible only as a superposition of contributions from large
galactic structures confused by intervening spiral arms along the
line of sight. This diculty is exactly the same we found in this
paper.
7. Conclusions
This is the first time that an UBVI CCD photometric study is
carried out in the region of the open cluster Trumpler 27 towards
l = 355
. In order to determine the properties of the stellar pop-
ulation, we have applied well known broad-band photometric
techniques. Our goal has been twofold: on one side to recognize
the main parameters of the cluster -if it exists- since several
observers in the past arrived at dierent conclusions and, on
the other side, to analyze the field star population present in the
region, since the same studies underlined the presence of a large
number of bright stars that have no relation withthe cluster itself.
An open cluster is, in essence, a decreasingsequence of grav-
itationally bound stellar masses well confined in a volume of
space. In the case of nearby open clusters, the analysis of proper
motions and radial velocities becomes a powerful tool to asses
the cluster membership on any star found in the field. As for dis-
tant clusters, the way to assess if a star belongs to it is by resting
on a spectrophotometric study focused on some sort of stellar
overdensity detected against the sky background. In the zone of
Trumpler 27 there is neither reliable proper motions -given the
distances of the stars in the region- nor appreciable star over-
density even using infrared data as shown in Fig. 11, upper left
panel.
From a spectrophotometric point of view we cannot confirm
or rule out definitely the existence of the cluster since results of
our analysis based on separating early type stars from field stars
are not conclusive at all. This is, CMDs showing the same pat-
tern can be constructed with stars at dierent distances from the
Sun neighborhood all the way to the Galactic center. Massive
stars of O-type, evolved B-types, M- and G- super-giants plus
two WR stars are found at all distances and widely separated -
angularly- from each other making it unlikely to connect them,
12
Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk
Fig.12. The path of reddening with distance according to the star spectral type. The approximate position and width of the inner
spiral arms is shown by dashed columns; the probable beginning of the Bulge is indicated by the arrow. Numbers in parentheses as
in Fig. 8.
from an evolutionary point of view. The dierent CMDs in opti-
cal and infrared are typical of very young star groups superposed
along the line of sight resembling, e.g., OB associations. This
could be the reason for the age spread detected by MDEW01
since the entire star group associated historically with Trumpler
27 is composed by stars which did not form in a same event and
do not share the same spatial location.
Basing on our data, it is dicult to assess the existence of
Trumpler 27 at the distance, with the members and with the
age derived in previous articles already cited. In addition, we
were unable to find in our diagrams a star sequence composed
by faint stars that could be related beyond doubt with potential
massive members of Trumpler 27. This is a serious obstacle
and we find it unlikely that photometry alone could solve the
question in the future. Certainly, the controversy may be settled
by means of an exhaustive spectroscopic study able to reach
also faint stars. An interesting point to interpret, in terms of
star formation too, is the lack of any evident HII region in the
zone as expected in a region of massive star formation. WR
and O-Type stars are usually injecting a tremendous amount of
ultraviolet photons in the surrounding gas which should be seen
in emission but there are no clear traces of emitting gas.
As far as the diuse young star population is concerned, in the
direction l = 355
, our star sample shows an even distribution.
This suggests that early type stars define a continuous superpo-
sition of stars with no density peaks from one arm to the next,
except for the prominent clump at the location of the Scutum-
Crux arm, as seen in Fig. 9.
The findings of this paper can be connected with the ones
from Carraro (2011) at l = 314
. Firm evidence was found there
of a young stellar population associated with Carina-Sagittarius
and Scutum-Crux arms. Moreover, the existence of an even star
distribution in the direction l = 314
, where arms are more open
than in the Galactic center direction, was found. This confirmed
the outcome of old (e.g. Muzzio and Levato 1980 and references
therein) and more recent (Carraro & Costa 2009, Baume et al.
2009) studies that provided robust optical evidences of their ex-
istence and their potential as spiral arm indicators.
However, in the present study which looks towards the cen-
ter of the Galaxy, four spiral arms are closely packed in only 5
kpc from the Sun. Therefore, the task of disentangling one struc-
ture from the next is more cumbersome, and would require the
support of an improved radio trigonometric parallax method to
measure accurate distances and proper motions of obscured star
forming regions in the Milky Way (Brunthaler et al. 2011).
Meanwhile, we believe it is of extreme value to continue
our program which aims at studying the inner disk stellar pop-
ulations using UBV and infrared photometry by extending it to
other absorption windows, as in the case of Trumpler 27. Young
stellar populations can be very well tracked using UBV photom-
etry and when combined with findings from radio observations,
a better picture of large structures can be achieved certainly.
Acknowledgements. G. Carraro expresses his gratitude to the Las Campanas
Observatory sta, and in particular to Patricio Pinto for the excellent support
during various observing runs. We also thank Sandy Strunk for reading carefully
the manuscript. G. Perren and R.A. V´azquez acknowledge the financial support
from the CONICET PIP1359. This research has made use of the NASA/IPAC
Infrared Science Archive, which is operated by the Jet Propulsion Laboratory,
California Institute of Technology, under contract with the National Aeronautics
and Space Administration.
We acknowledge useful and constructive comments from an anonymous ref-
eree which allowed us to improve the manuscript.
References
Ahumada, J. A.; Giorgi, E. E.; Solivella, G.; V´azquez, R. A. 2011 MNRAS 415,
1355
Baume, G., Carraro, G., Momany, Y., 2009, MNRAS, 398, 221
Bakker R., The P.S., 1983, A&AS 52, 27
Bitran M., Alvarez H., Bronfman L., May J., and Thaddeus P., 1997, A&ASS
125, 99
13
Gabriel Perren et al.: Photometric distances to young stars in the inner galactic disk
Brunthaler, A. et al. 2011 AN 332, 461
Carraro, G., Costa, E., 2009, A&A, 493, 71
Carraro, G., V´azquez, R.A., Moitinho, A., Baume, G., 2005, ApJ, 630, L153
Carraro G., V´azquez, R. A., Costa, E., Perren, G., Moitinho, A., 2010 ApJ 718,
683
Carraro, G., 2011 A&A 536, 101
Churchwell E. et al. 2009, PASP 121, 230
Conti, P. S., Vacca, W. D. 1990 AJ, 100, 431
Dame T.M., and Thaddeus P., 2008 ApJ, 683, L143
Hamuy, M., Folatelli, G., Morrel, N.I., Phillips, M.M., et al., 2006, PASP, 118, 2
Hiltner W.A., Johnson H.L., 1956 ApJ 124, 367
Green J. A., Caswell J. L., McClure-Griths N. M., Avison A., Breen S. L.,
Burton M. G., Ellingsen S. P., Fuller G. A., Gray M. D., Pestalozzi M.,
Thompson M. A., Voronkov M. A., 2011 ApJ 733, 27
Knapp G.R., Sutin B.M., Ellison B.N., Keene J.B., Leighton R.B., Masson C.R.,
Steiger W., Veidt B., Young K., 1989 ApJ 336, 822
Landolt A. U., 1992 AJ 104, 340
L´epine J.R.D., Roman-Lopes A., Abraham Z., Junqueira T.C., Mishurov Yu. N.,
2010 MNRAS,
Lundstr¨om I, Stenholm B., 1984 A&AS 58, 163
Ma´ız-Apella´ıniz J., Walborn N. R., Galu´e H., Wei, L. H. 2004 ApJS 151, 148
Marraco, H. G.,Rydgren, A. E. 1981, AJ, 86, 62
Massey P., DeGioia-Eastwood K., Waterhouse E., 2001 AJ 121, 1050
McSwain, M.V. & Gies, D.R. 2005 ApJSS 161, 118
Minniti, D., Lucas, P. W., Emerson, J. P., Saito, R. K., Hempel, M., Pietrukowicz,
P., Ahumada, A. V., et al. 2010, NewA 15, 433
Moat A. F. J., FitzGerald M. P., Jackson P.D. 1977, ApJ 215, 106
Moitinho, A., V´azquez, R.A., Carraro, G., Baume, G., Giorgi, E.E., Lyra, W.,
2006, MNRAS, 368, L77
Moni Bidin, C., Mauro, F., Geisler, D., Minniti, D., Catelan, M., Hempel, M.,
Valenti, E., Valcarce, A. A. R., et al. 2011, A&A, 535, 33
Muzzio, J.C., Levato H. O. 1980 PASP 92, 36
Neckel Th., Klare G., 1980 A&ASS 42, 281
Parthasarathy M., Reddy B.E., 1993 ASIB 21, 619
Patat F., Carraro G. 2001 MNRAS 325, 1591
Reed, C. B., 2003, AJ, 125, 2531
Russeil, D., 2003, A&A 397, 133
Samus, N. N., Kazarovets, E. V., Kireeva, N. N., Pastukhova, E. N., Durlevich,
O. V., 2010 Odessa Astronomical Publications, 23, 102
Schmidt-Kaler, T. 1982, Landolt-B¨ornstein, Group VI, Vol. 2b, Stars and Star
Clusters (Berlin: Springer), 15
Skrutskie, M. F., Cutri, R. M., Stiening, R., Weinberg, M. D., Schneider, S.,
Carpenter, J. M., Beichman, C., Capps, R., Chester, T., Elias, J., et al. 2006
AJ 131, 1163
Smith L.F., 1968 MNRAS 140, 409
Stetson P. B., 1987 PASP 99, 191
The P.S., Stokes N. 1970, A&A 5, 298
Vall´ee, J., 2005, AJ, 130, 569
van der Hucht K. A., 2001, New Astronomy Reviews 45 135-232 The VIIth
catalogue of galactic Wolf-Rayet stars.
van Genderen A.M., 1980 A&A 88, 77
V´azquez, R. A., May, J., Carraro, G., Bronfman, L., Moitinho, A., Baume, G.,
2008, ApJ, 672, 930
Zacharias, N., Finch, C., Girard, T., Hambly, N., Wyco, G., Zacharias, M. I.,
Castillo, D., Corbin, T., et al. 2010, AJ 139, 2184
14